The load circuit of the present invention is disclosed in PCT/JP96/00866. PCT/JP96/00866 is incorporated herein by reference.
In a conventional load drive circuit, the normal shutdown switch and the emergency shutdown switch are disposed in series in a load power supply circuit. An ON/OFF control signal for the emergency shutdown switch which is based on the occurrence of a drive signal for a load, is applied to the emergency shutdown switch via an off-delay circuit. Moreover, an ON/OFF control signal for the normal shutdown switch is based on the occurrence of the drive signal is applied to the normal shutdown switch via an on-delay circuit.
A description will now be given of the operation of the conventional circuit.
The ON/OFF control signal for the emergency shutdown switch is generated from the off-delay circuit simultaneously with the application of the drive signal. Meanwhile the ON/OFF control signal for the normal shutdown switch is generated after an on-delay time delay of the on-delay circuit from the application of the drive signal. When the drive signal is terminated, the ON/OFF control signal for the emergency shutdown switch is stopped after an off-delay time delay. This off-delay time delay is determined from the moment of termination of the drive signal. Meanwhile, the ON/OFF control signal for the normal shutdown switch is stopped simultaneously with the termination of the drive signal. Consequently, when conditions are normal, the emergency shutdown switch does not directly switch the current ON or OFF. Rather, the load current is switched ON or OFF by means of the normal shutdown switch. Furthermore, in the case where the load current cannot be interrupted due to contact point fusion in the normal shutdown switch, the emergency shutdown switch is forcibly switched off in order to interrupt the load current thereby forcibly stopping operation of the load.
With the conventional circuit however, a fusion or short circuit fault of the normal shutdown switch cannot be determined unless the drive signal is off. However, under conditions when the drive signal is being generated and the normal shutdown switch is on, the capacity to interrupt the load current may be actually lost due to, for example, a fusion fault. Moreover, in the case where the normal shutdown switch has a short circuit fault and the load current is interrupted by the emergency shutdown switch, the off-delay time control signal from the off-delay circuit continues from the point in time of termination of the drive signal. Thus the OFF operation of the emergency shutdown switch is delayed. Subsequently, the OFF response of the load current is delayed. This is inevitable with a switch circuit where the current is not directly ON/OFF switched by the emergency shutdown switch.
The object of the present invention is to overcome these and other disadvantages of the conventional load drive circuit. The present invention provides a load circuit which can check if the normal shutdown switch can switch the current OFF during load drive and without stopping load drive.